Integrated flash memory systems and methods for load compensation

ABSTRACT

Systems and methods are disclosed including features that compensate for variations in the magnitude of supply voltages used in memory arrays. According to some aspects, compensation circuits may provide a tunable current-limiting load for data columns, where the load can be tuned to dynamically compensate for variations in supply voltage. In certain aspects, a compensation circuit may employ an operational amplifier configured as a voltage follower. The voltage follower compensates for any variations in supply voltage, forcing a constant voltage drop across the load element(s), thus maintaining a constant load. Other circuits may also be included, such as precharge circuits, clamp circuits, buffer circuits, trimming circuit, and sense amplifier circuits with sensed body effect. System-On-Chip integrated system aspects may include a microcontroller, a mixed IP, and a flash memory system having functionality and blocks that interface and interoperate with each other for load compensation.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a continuation of application Ser. No. 11/655,901, filed Jan.19, 2007, published as US2008/0175062A1, now U.S. Pat. No. 7,660,161,which are incorporated herein by reference in entirety.

BACKGROUND

1. Field

This invention relates generally to flash memory arrays that are used inintegrated flash systems or system-on-chip (SOC) microcontroller flashsystems. More specifically, this invention relates to load compensationin multilevel flash memory arrays.

2. Description of Related Information

As information technology progresses at an unprecedented pace, the needfor information storage increases proportionately. Accordingly, the nonvolatile information in stationary or portable communication demandshigher capability and capacity storage. One approach to increasing theamount of storage is by decreasing physical dimensions of the stored bit(e.g., memory cell) to smaller dimensions such as nanocell technology.Another approach is to increase the storage density per bit. The secondapproach is known as digital multilevel nonvolatile storage technology.A sense amplifier reads the content of a memory cell by comparison toreference levels. As more bits are stored in a multilevel memory cell,the voltage separation of reference levels decreases.

Variations in many parameters, such as the magnitude of supply voltageVDD, can result in inaccurate data readings and other errors as voltageseparation decreases and corresponding sensitivity to such variationsincreases. Accordingly, it is often desirable to develop systems andmethods of compensating for these variations, so as to reducefluctuations in the operating parameters of memory arrays and improvethe accuracy with which data is written to, or read from, multilevelmemory cells.

SUMMARY

The innovations herein can be implemented in numerous ways that overcomethe above drawbacks by use of circuitry that compensates for variationsin load or performs other load-related aspects to enable more accuratearray operation.

A multilevel memory system consistent with aspects of the innovationsherein may comprise a memory sensing circuit connected to the multilevelmemory cell and having a voltage supply terminal, a load elementconnected between the voltage supply terminal and the multilevel memorycell, the load element providing a load for the memory sensing circuit,as a function of a voltage drop across at least a portion of the loadelement and a current through the load element, and a load compensationcircuit connected to the load element and configured to maintain theload by compensating for a load variation of the load.

Another multilevel memory system consistent with aspects of theinnovations herein may comprises a memory sensing circuit connected tothe multilevel memory cell and configured to generate an output voltagesignal corresponding to data stored in the multilevel memory cell, and acomparator circuit having a first transistor and a second transistor,the comparator circuit configured to compare the output voltage signalto a reference voltage signal according to a difference between a bulkgain of the first transistor and a bulk gain of the second transistor,the gain of the first transistor determined according to the referencevoltage signal, and the gain of the second transistor determinedaccording to the output voltage signal.

A system-on-chip controller memory system consistent with aspects of theinnovations herein may comprise a microcontroller, a mixed signal IPsuitable for interface with the microcontroller, a memory system with asensing circuit connected to the memory cell and having a voltage supplyterminal, a load element connected between the voltage supply terminaland the memory cell, the load element providing a load for the memorysensing circuit, the load provided according to a voltage drop across atleast a portion of the load element, and a current through the loadelement, the load having a load variation, and a load compensationcircuit connected to the load element and configured to maintain theload by compensating for the load variation.

Another system-on-chip controller memory system consistent with aspectsof the innovations herein may comprise a microcontroller, a mixed signalIP, a memory system including a voltage supply terminal and a sensingcircuit coupled to a memory cell, a load element connected between thevoltage supply terminal and the memory cell, the load element connectedto a bulk terminal of a transistor of the memory sensing circuit, and aload compensation circuit connected to the load element and configuredto maintain the load by compensating for a load variation.

Other aspects and advantages of the invention will become apparent fromthe following detailed description and claims taken in conjunction withthe accompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference should be made tothe following detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1A is a block diagram illustrating one integrated system-on-chip(SOC) microcontroller memory system consistent with aspects of thepresent invention.

FIG. 1B is a block diagram illustrating a digital multilevel bit memorysystem consistent with aspects of the present invention.

FIG. 2 is a schematic diagram illustrating a conventional sensing systemconsistent with aspects of the present invention.

FIGS. 3A-3F are schematic diagrams illustrating various supply voltagecompensation circuits configured in accordance with aspects of thepresent invention.

FIG. 3G is a schematic diagram illustrating a precharge circuitconfigured in accordance with aspects of the present invention.

FIGS. 3H-3K illustrate buffer circuits configured in accordance withaspects of the present invention.

FIGS. 3L-3N are schematic diagrams illustrating further supply voltagecompensation circuits configured in accordance with aspects of thepresent invention.

FIGS. 4A-4B are schematic diagrams of further supply voltagecompensation circuits in which load transistors operate in theirsaturation regions, consistent with aspects of the present invention.

FIGS. 5A-5B are schematic diagrams of further supply voltagecompensation circuits in which load transistors operate in theirsaturation regions, consistent with aspects of the present invention.

FIGS. 6A-6B are schematic diagrams illustrating comparator circuitsemploying body effects, in accordance with aspects of the presentinvention.

Like reference numerals refer to corresponding parts throughout thedrawings.

DETAILED DESCRIPTION OF EMBODIMENTS

In a first example, aspects of the present innovations relate to memorysystems that compensate for variations in the magnitude of supplyvoltages used in multilevel memory arrays. Here, a compensation circuitprovides a tunable current-limiting load for data columns, where theload can be tuned to dynamically compensate for variations in supplyvoltage. In some embodiments, a PMOS transistor acts as acurrent-limiting load, and the compensation circuit employs anoperational amplifier configured as a voltage follower, for maintaininga specified voltage drop across the PMOS transistor. The voltagefollower compensates for any variations in supply voltage, forcing aconstant voltage drop across the PMOS, and thus maintaining a constantload despite any variations in supply voltage. The invention alsoencompasses the use of other circuits, such as precharge circuits forprecharging lines like reference voltage lines, clamp circuits formaintaining a minimum voltage in data columns, buffer circuitsdecoupling noise from data signals, trimming circuit for trimming datasignals, and comparator circuits operating according to the body effectof their component transistors.

FIG. 1A is a block diagram illustrating an exemplary integratedsystem-on-chip (SOC) microcontroller memory system 10 consistent withone or more aspects related to the innovations here. The SOC controllermemory system 10 includes a digital multilevel bit memory array system100, a microcontroller 20, a mixed IP block 30 (i.e. a circuit or blockof circuitry that may be devoted to a particularized application).System bus includes address, data, control, and mixed signal bus (thatincludes digital control, clock lines, power lines, and analog signallines). The digital multilevel bit memory array system 100 will bedescribed in details in FIG. 1B, served as a code and data storage forthe system 10. The microcontroller 20 is such as an 8-bitmicrocontroller such as industry standard 8051, a RISC core, or a 32 bitARM controller. The mixed IP such as 10-bit ADCs (Analog-to-DigitalConverter) and/or 10-bit DACs (Digital-to-Analog Converter), XTAL(crystal oscillator), PLL (phase lock loop), voltage regulators, bandgapreference, Power-On-Reset (POR) circuit, VDD-detector (for powerbrown-out detection, detecting invalid VDD level to avoid invalid chipoperation), etc. In one embodiment, the flash system 100 interfaces withthe Mixed IP 30 such as using the ADC and DAC to generate high voltageprogram, erase, and sensing biases (such as described in U.S. Pat. No.6,788,608 by Tran et al), and using clock signal for charge pumping andfor program, erase, read, and IO burst algorithm control. In anotherembodiment, the microcontroller 20 provides security function for theflash system 100 (such as chip password protection, memory sector tagbit, user ID code, memory block or sector non-volatile or volatileprotection) and error detection and correction (such as parity check,Hamming code, cyclic codes). In another embodiment, the microcontroller20 and Mixed IP 30 uses the high voltage output (typically 10-20V) ofthe charge pump of the flash memory 100 for such as providing voltageoverdrive for switches and big drivers to reduce impedance. In anotherembodiment, the microcontroller 20 and/or the memory system 100 uses thevoltage regulator from the mixed IP 30 to provide regulated power supplyfor the microcontroller 20 and/or the memory system 100. In anotherembodiment, the microcontroller 20 and/or the memory system 100 uses thePower-On-Reset and/or VDD-detector from the mixed IP 30 to provide poweron reset for the microcontroller 20 and/or the memory system 100. Inanother embodiment, the microcontroller 20 and/or the memory system 100uses the VDD-detector from the mixed IP 30 to provide power supply leveldetection function for the microcontroller 20 and/or the memory system100. In another embodiment, the memory system 100 provides its own PORand/or VDD-detector to the microcontroller 20 and/or the mixed IP 30.

FIG. 1B is a block diagram illustrating a exemplary digital multilevelbit memory array system 100 consistent with one or more aspects relatedto the present invention. The digital multilevel bit memory array system100 of FIG. 1B is comprised of a memory array 101 that includes aplurality of memory cells (not shown) and a reference array 106 thatincludes a plurality of reference memory cells (not shown). An N bitdigital multilevel cell is defined as a memory cell capable of storingthe 2^(N) levels. The reference array 106 is used as a reference systemof reference voltage levels to verify the contents of the memory array101. In another embodiment, the memory array 101 may include referencememory cells for storing the reference voltage levels.

In one embodiment, the memory array 101 and the reference array 106include a drain side injection flash technology, which uses lower powerin hot electron programming, and efficient injector basedFowler-Nordheim tunneling erasure. The programming may be done byapplying a high voltage on the drain of the memory cell, a bias voltageon the control gate of the memory cell, and a bias current on the sourceof the memory cell. The programming in effect places electrons on thefloating gate of memory cell. The erase is done by applying a highvoltage on the control gate of the memory cell and a low voltage on thedrain and/or source of the memory cell. The erase in effect removeselectrons from the floating gate of memory cell. The verify (sensing orreading) is done by placing the memory cell in a voltage mode sensing,e.g., a bias voltage on the source, a bias voltage on the gate, a biascurrent coupled from the source (bitline) to a low bias voltage such asground, and the voltage on the source is the readout cell voltage VCELL.The bias current may be independent of the data stored in the memorycell. In another embodiment, the verify (sensing or reading) is done byplacing the memory cell in a current mode sensing, e.g., a low voltageon the source, a bias voltage on the gate, a load (resistor ortransistor) coupled to the source (bitline) from a high voltage supply,and the voltage on the load is the readout voltage. In one embodiment,the array architecture and operating methods may be consistent withthose disclosed in U.S. Pat. No. 6,282,145, entitled “Array Architectureand Operating Methods for Digital Multilevel Nonvolatile MemoryIntegrated Circuit System” by Tran et al., and/or U.S. Pat. No.7,139,196, entitled “Sub-Volt Sensing For Digital Multilevel Memory” byTran, which are incorporated herein by reference in their entirety.

The multilevel memory cells of the memory array 101 may be arranged invarious ways, such as in rows and columns or in segments. Variousaddressing schemes may be used which organize the memory cells intobytes, pages or other arrangements.

The digital multilevel bit memory array system 100 further includes an xdecoder 120, a y decoder 110, an address controller 162, a senseamplifier circuit 111, and an intelligent input/output interface 196.The y decoder 10 controls bitlines (not shown) coupled to columns inmemory cells and the reference voltage cells, during a write, read (orverify), and erase operations. The sense amplifier 111 senses the readdata which is provided to the I/O interface 196. The I/O interface 196also buffers input into the memory array system 100. The sense amplifier111 also senses the read data and verifies the read data against inputdata during memory programming or erasing.

In response to address and other control signals, the address controller162 decodes the address signal and controls page, byte, segment or otheraddressing for the x decoder 120 and the y decoder 110. The x decoder120 selects a row or a block of rows in the arrays 101 and 106 based onthe signals from the address controller 162 and provides precisemultilevel bias values over temperature, process, and power supply usedfor consistent single level or multilevel memory operation for thememory array 101.

The system 100 includes a logic controller 163 to control various chipfunctionality and to interface with the Mixed IP 30 and theMicrocontroller 20.

The system 100 includes a voltage/current generator V&I-GEN 175 togenerate biases for program, erase and read operation. The V&I-GEN 175may interface with the Mixed IP 30 such as using the ADC,DAC for biasesand pulse-shape generation and PLL and XTAL for clock sources.

The system 100 includes a flash built-in self test, FBIST 181, that isused to test on-chip non-volatile program and erase function, redundancyself repair, addressing uniqueness, defect open/short screening forbitline, wordline or sourceline, memory cell terminal leakage test (suchas bitline, wordline or sourceline), power consumption test (standby oractive), disturb screening, infant mortality screening, marginscreening, at-speed test, etc.

The system 100 further includes power related circuits (not shown), suchas band gap voltage generators, charge pumps, voltage regulators, andpower management systems, and other control circuits (not shown) such asvoltage algorithm controllers.

The system 100 may execute various operations on the memory array 101.An erase operation may be done to erase all selected multilevel cells byremoving the charge on selected memory cells according to the operatingrequirements of the non-volatile memory technology used. A data loadoperation may be used to load in a plurality of bytes of data to beprogrammed into the memory cells, e.g., 0 to 512 bytes in a page. A readoperation may be done to read out in parallel a plurality of bytes ofdata if the data (digital bits), e.g., 512 bytes within a page, storedin the multilevel cells. A program operation may be done to store inparallel a plurality of bytes of data in (digital bits) into themultilevel cells by placing an appropriate charge on selected multilevelcells depending on the operating requirements of the non-volatile memorytechnology used. The operations on the memory may be, for example,consistent with the operations described in U.S. Pat. No. 6,282,145,incorporated herein by reference above.

FIG. 2 is a schematic diagram illustrating a conventional sensing system200. The conventional sensing system 200 comprises a reference column201, a plurality of data columns 202-0 through 202-N, and a plurality ofcomparators 203-0 through 203-N. The reference column 201 comprises areference memory cell 211, an NMOS transistor 212 and a PMOS transistor215. A bitline resistor 213 is shown to indicate resistance on thebitline. A bitline capacitor 214 is shown to indicate capacitance on thebitline. The reference column 201 provides a voltage reference on thereference line 204 which is applied to a first input of each of thecomparators 203-0 through 203-N. Each data column 202 comprises a datamemory cell 221, an NMOS transistor 222 and a PMOS transistor 225. Abitline resistor 223 is shown to indicate resistance on the bitline. Abitline capacitor 224 is shown to indicate capacitance on the bitline.Each of the data columns 202-0 through 202-N provides a data outputvoltage to a second input of a respective comparator 203-0 through 203-Nso that the comparator 203 provides an output indicative of the storeddata in the corresponding data column 202.

The conventional sensing system 200 has mismatches within the systembecause of differences in the PMOS transistors 215 and 225 (such asthreshold voltage or transconductance beta) that provide loads for therespective reference column 201 and the data column 202. Further,mismatches can occur due to variations in VDD lines, such as voltagedrops across VDD lines due to the resistance of these lines. Evenfurther, the comparators 203 have a mismatch in their inputs. Thesemismatches may lead to inaccurate reads of the data cells 221. Moreover,the bitlines may have a mismatch in capacitances that may lead toinaccurate reads, especially in dynamic reads. Other mismatches may comefrom layout, such as voltage drop along power lines or interconnectlines.

The mismatches may cause a difference dVo in voltage between outputs ofthe load transistors 215 and 225 due to the PMOS transistors mismatch ofthe threshold voltage VT, beta mismatch, or voltage drop mismatch, suchas VDD, bias current Ibias, or voltage bias Vbias. The differencevoltage dVo is typically between 20 and 50 millivolts, but can behigher, e.g., a few hundred millivolts for current large chips.

One aspect of the present invention involves compensation for thesemismatches, especially those that can occur due to variations and/orfluctuations in the PMOS transistors 215, 225, and in the supply voltageVDD. To that end, the sense amplifier 111 (FIG. 1) may include thesensing systems and subsystems of FIGS. 3-4. FIG. 3A illustrates oneexample of a supply voltage compensation circuit configured inaccordance with the present invention. In contrast with conventionalsystems that include only a diode connected PMOS transistor 225providing a load, a compensation circuit 300 is provided, to compensatefor variations in the load that the PMOS transistor 225 provides. Thecompensation circuit 300 includes an operational amplifier 302, avoltage source 304, and a current source 306. The current source 306 isset up to pass a specified amount of current through the PMOS transistor225R (tuned element/transistor or reference element/transistor). Thetransistors PMOS 225 are tuned to the transistor PMOS 225R, meaning itsgate voltage is biased to be the same as that of the transistor PMOS 225and its size is of a predetermined ratio, e.g. 2:1, to that of thetransistor PMOS 225R. The operational amplifier 302 is connected at itsoutput terminal tuning node VBA to the gate terminal of PMOS transistor225R, while its inverting terminal (“−”) is connected to voltage source304 and its noninverting terminal (“+”) is connected to the drainterminal of the PMOS transistor 225R. It can thus be seen that theoperational amplifier 302 acts as a voltage follower, maintaining avoltage drop across the drain terminal and the source terminal of thePMOS transistor 225R according to the voltage supplied to its invertingterminal by the voltage source 304.

As the drain-source voltage is fixed by the operational amplifier 302,and the current through the PMOS transistor 225R (hence the PMOStransistor 225) is fixed by the current source 306, the compensationcircuit 300 sets the “effective resistance,” or load, provided by thePMOS transistor 225 at a predetermined value equal to the drain-sourcevoltage divided by the current from the current source 306. That is, thecompensation circuit 300 maintains a constant load, eliminating loadvariations by maintaining a constant drain-source voltage across thePMOS transistor 225 and a constant current through the same PMOStransistor 225. More specifically, by acting as a voltage follower, thecompensation circuit 300 compensates for fluctuations in VDD or otherproperties that cause load variations such as process or temperaturevariation, maintaining a constant voltage drop across the load elementeven when VDD varies.

This same result can also be achieved by other circuit configurations,of the invention. FIG. 3B illustrates another example of a supplyvoltage compensation circuit configured in accordance with the presentinvention. Here, compensation circuit 400 includes an operationalamplifier 402 with its output tuning node VBA electrically connected tothe gate terminal of PMOS transistor 225R, its inverting terminalconnected to resistor 404 which is in turn connected to a voltage source410, and its noninverting terminal connected to the drain of PMOStransistor 225R. The noninverting and inverting terminals are alsoelectrically connected to transistors 406 and 408 respectively, whichare configured as a current mirror, for example current in transistor408 is fixed at a mirror ratio, e.g., 2:1 or 1:1, to current intransistor 406. In operation, the operational amplifier 402 functions asa voltage follower, maintaining a constant drain-source voltage in thePMOS transistor 225R according to the voltage at the “−” terminal ofoperational amplifier 402. This voltage is, in turn, determinedaccording to the magnitudes of the drain voltage and the resistance ofthe resistor 404. The current through this resistor 404 is mirroredthrough the PMOS transistor 225R by the current mirror (i.e.,transistors 406, 408). Accordingly, it can be seen that the compensationcircuit 400 produces a predetermined, constant load according to aspecified voltage drop across PMOS 225R and PMOS 225 and a specifiedcurrent through PMOS 225R and PMOS 225, by simply selecting anappropriate voltage level at terminal 410 and an appropriate resistancevalue for the resistor 404.

FIG. 3C illustrates yet another example of a supply voltage compensationcircuit. The compensation circuit 500 includes an operational amplifier502 with its output tuning node VBA electrically connected to the gateterminal of PMOS transistor 225R and transistor PMOS 225, its invertingterminal connected to resistor 504 which is in turn connected to avoltage source 510, and its noninverting terminal connected to the drainof PMOS transistor 225. The inverting and noninverting terminals arealso electrically connected to current sources 506, 508, which functionsimilarly to the current mirror with a mirror ratio M:N of FIG. 3B,specifying a fixed amount of current that flows through the resistor 504and PMOS 225R (and hence PMOS 225). This in turn specifies the voltagedrop across PMOS 225R. In this manner, the compensation circuit 500produces a constant load for the PMOS 225, by specifying a currentthrough the PMOS 225 and by maintaining a constant voltage drop acrossPMOS 225 according to the voltage at the inverting terminal of theoperational amplifier 502 (which is determined by the voltage at thevoltage source terminal 510, the resistance of resistor 504, and thecurrent source 506).

While the above described compensation circuits are discussed in thecontext of their ability to compensate for changes in VDD voltage, oneof ordinary skill in the art will realize that the compensation circuitsof the invention can compensate for changes in many different propertiesof memory cell circuitry. For example, in addition to the abovedescribed ability to compensate for changes in VDD voltage, the abovecircuits maintain a constant voltage drop across the load element PMOS225, which can compensate for factors such as load changes due totemperature variations in PMOS 225 that alter its electrical properties.The compensation circuits can also compensate for load variations causedby variations in the fabrication of the load elements, resulting fromslightly differing device dimensions and properties. By forcing aconstant voltage drop despite these variations, the compensationcircuits of the invention can correct for any variations in load,maintaining the voltage drop across the load element(s) at apredetermined value, thus maintaining the load on the data column 202 ata predetermined value.

The innovations herein also include the use of other circuits besidesthe supply voltage compensation circuits described above. For instance,the invention includes the use of a precharge circuit, for prechargingvoltage lines and thereby reducing settle times in data storage andretrieval. FIG. 3D illustrates one example of a precharge circuitconsistent with one or more aspects of the current invention. Aprecharge circuit 600 includes a voltage source terminal 602 andresistors 604-608. The resistors 604-608 are connected in series betweenthe voltage source terminal 602 and a ground terminal, with the nodebetween resistors 604 and 606 also connected to VREF line 204 throughswitch 610. In operation, when switch 610 is closed, a voltage can beapplied at the voltage source terminal 602 to transmit a charge to VREFline 204, helping to precharge the line 204. This allows the VREF line204 to reach its desired voltage levels faster, thus reducing settletimes and read/write times. When a precharge is not needed, switch 610can be opened to prevent the circuit 600 from precharging the VREF line204. Here, as the resistors 604, 606 are configured as a voltagedivider, it can be recognized that the voltage at the node betweenresistors 604 and 606 depends on the ratio of the resistance of resistor604 to that of resistor 606. Accordingly, the magnitude of theprecharge, and the speed at which it can be applied to VREF line 204,can be adjusted according to the ratio of resistances of resistors 604and 606. Also shown are switches across terminals of comparators to beclosed during precharging.

In addition to precharge circuits, the innovations herein include theuse of clamp circuits for maintaining minimum voltages within datacolumns 202. FIG. 3E illustrates one example of a clamp circuitconsistent with one or more aspects of the invention. Here, clampcircuits 700 have a PMOS transistor 702 with a source terminal connectedto a voltage source terminal 704, and gate and drain terminals connectedto the VOUT of data columns 202. In operation then, the PMOS transistor702 is kept in the on state by the voltage from the voltage sourceterminal 704, thus maintaining a minimum voltage at the VOUT terminalsand keeping data column voltages from falling to zero. The maintainingof such minimum voltages is often desirable, as zero voltages withindata columns 202 can often result in excessive time taken betweenread/write operations, as data column voltages must then be boosted backup before a read operation can commence.

The innovations also include the use of buffer circuits for decouplingnoise and loading from the comparators 203. In this embodiment, a buffercircuit 800 includes a voltage source terminal 802, a native NMOStransistor 804, and an additional transistor 806. The native NMOS 804has its drain connected to the voltage source terminal 802, its gateconnected to VOUT, and its source connected to the drain of theadditional transistor 806. The transistor 806 also has a grounded sourceterminal. In operation then, the VOUT signal is applied to the gate ofthe native NMOS 804 so as to pass a corresponding signal from thevoltage source terminal 802 to the comparator, this corresponding signalindicative of data stored in the data column 202. However, noise (andthe loading) from the comparators 203 is blocked by the NMOS 804, and isnot seen at the data column 202. Thus, the buffer circuit 800 allowsdata signals out to the comparator 203, while effectively blocking noiseback from the comparator 203 to the data column 202.

It should be noted that the invention is not limited to the use ofprecharge, clamp, or buffer circuits in isolation. Rather, the inventionencompasses the use of any or all of these circuits, in any combination.As one example, FIG. 3G illustrates the use of a precharge circuit 600,clamp circuit 700, and buffer circuit 800 simultaneously. It should alsobe noted that the invention is not limited to the use of the componentsdescribed above, but rather encompasses the use of any suitable devicesor components. For example, the clamp circuit 700 need not bespecifically limited to the use of a PMOS 702, but rather can employ anNMOS transistor 900, as shown in FIG. 3G. While one of ordinary skill inthe art will realize that the various different components are desirablefor different applications, it can be noted here that the NMOStransistor 900 can be used in place of the PMOS 702 when, for instance,it is desirable to allow the voltage VB2, applied at the gate of NMOS900, to vary by a greater amount than would the voltage VB2 applied atthe gate of PMOS 702.

The innovations also include the use of trimming circuits for trimmingthe voltage signal out from the data columns 202 to the comparators 203.This trimming is often desirable in allowing room for offsets, margintesting (margin refers to an operating range within which a cell voltageor current belongs to), and the like. FIGS. 3H-3K are variousillustrative embodiments of trimming circuits constructed in accordancewith the invention. In FIG. 3H, a trimming circuit 1000 includes threeresistors 1002-1006 connected in series, with the first resistor 1002connected between the output of buffer circuit 800 (i.e., the datasignal to comparators 203) and second resistor 1004, the second resistor1004 connected between first resistor 1002 and third resistor 1006, andthird resistor 1006 connected between second resistor 1004 and a groundterminal. When switch 1008 is closed and switches 1010, 1012 are open,the resistors 1002-1006 sink current, dropping the voltage of the datasignal to comparators 203. Various ones of the switches 1008-1012 can beopened, and the remainder closed, so as to drop the voltage out to thecomparators 203 to various desired levels. This allows any desiredamount of trimming to be applied to the signal out from the data columns202 to the comparators 203, for any desired offset or other applicationsuch as margin testing.

It is noted that while FIG. 3H illustrates a trimming circuit 1000employed in conjunction with a buffer circuit 800, the trimming circuitsof the invention need not necessarily be so used. Rather, the trimmingcircuits of the invention can be used in conjunction with any, all, ornone of the precharge circuit 600, clamp circuit 700, and buffer circuit800.

It is also noted that the invention is not limited to the trimmingcircuits shown in FIG. 3H, but rather encompasses any circuit fortrimming the output voltage of a data column 202. FIG. 3I illustratesone such alternative trimming circuit, where the trimming circuit isincorporated into a buffer circuit. Here, buffer transistor 804 isconnected between voltage terminal 802 and resistors 1100-1104, whichare connected in series similar to resistors 1002-1006. Here, theresistors 1100-1104 are connected between the source of NMOS 804, at oneend, and transistor 806, at the other end. Even though the resistors1100-1104 are incorporated within buffer circuit 800, it can be seenthat the resistors 1100-1104 sink current hence causing a voltage dropacross them, trimming the voltage at the source of NMOS 804, and furthertrimming the voltage at each successive output 1106-1110.

FIG. 3J illustrates another alternative trimming circuit consistent withone or more aspects of the present innovations. Here, a voltage terminal802 and buffer transistor 804 provide buffering as described above.Additionally, transistors 1200-1206 are connected in parallel betweenthe source of NMOS 804 and a ground terminal. Switches 1208-1214 areplaced between the source of NMOS 804 and the transistors 1200-1206 asshown, to allow transistors 1200-1206 to be selectively engaged. Inoperation, the switches 1208-1214 are selectively closed to engagedesired transistors 1200-1206, each of which can sink current and dropthe voltage at the source of NMOS 804 by a predetermined amount (i.e.,the known voltage drop across whichever ones of the transistors1200-1206 are engaged, sized such as in a binary orincremental-by-one-unit value fashion,). The transistors 1200-1206 canbe chosen to provide differing voltage drops and thus differing trimmingamounts, allowing switches 1208-1214 to be adaptively closed accordingto the amount of trimming desired.

FIG. 3K illustrates another alternative trimming circuit consistent withone or more aspects of the present innovations. Here, the VOUT terminalis not connected to an NMOS 804. Instead, the VOUT terminal is connectedto the gate terminals of transistors 1300-1306, which are in turnconnected in parallel between a voltage source terminal 1308 and aVOUTBUF terminal providing output voltage to the comparator 203. Anothertransistor 1310 is connected between VOUTBUF and ground. Switches1312-1318 are provided between the transistors 1300-1306 and VOUTBUF,and can be closed to selectively engage their corresponding transistors1300-1306. Accordingly, when ones of the transistors 1300-1306 areengaged and a VOUT signal is applied to their gate terminals, engagedtransistors 1300-1306, sized such as in a binary or incremental-by-onefashion, apply a voltage to the VOUTBUF line according to the magnitudeof the voltage from voltage source terminal 1308 and the voltage dropacross those transistors 1300-1306 that are engaged. The magnitude ofthe voltage from the voltage source terminal 1308, and the voltage dropacross transistors 1300-1306, can thus be chosen so as to yield aVOUTBUF signal that is smaller than the VOUT signal, effectivelyresulting in a trimmed VOUT signal.

As above, it should be noted that trimming circuits consistent with thepresent innovations can be employed both alone and in conjunction withother circuits and systems of the invention. For instance, the trimmingcircuits of FIGS. 3H-3K can be employed alone, or along with one or moreof the precharge circuit 600, clamp circuit 700, and buffer circuit 800described above.

The compensation circuits of the invention can also employadjustable-level resistors in various aspects consistent with thepresent innovations. More specifically, adjustable-level resistors canbe used to provide varying loads in data columns 202, where the loadscan be adjusted to compensate for variations in other parameters. Forexample, FIG. 3L illustrates a supply voltage compensation circuit inwhich the PMOS 225R (tuned element/transistor) connected to adjustableresistors 1400, 1402. The adjustable resistors 1400-1402 are connectedin series between the drain terminal of PMOS 225R and the ground. Inoperation, the resistances of resistors 1400-1402 can be varied so as toprovide a varying resistive load for the data columns 202. Inparticular, the resistance of resistors 1400-1402 can be varied, forexample as a binary or incremental-by-one-unit value by CMOS switchmuxing, so as to compensate for variations in VDD that change thevoltage seen at the source of PMOS 225R. That is, the resistances ofresistors 1400-1402 can be tuned so that the voltage drop acrossresistors 1400-1402 compensates for variations in the voltage at thesource of PMOS 225R.

FIG. 3M illustrates another embodiment of a supply voltage compensationcircuit employing adjustable-level resistors consistent with one or moreaspects of the present innovations. Compensation circuit 1500 is similarto compensation circuit 500 of FIG. 3C, in that it includes anoperational amplifier 1502 that is configured as a voltage follower formaintaining a constant drain-source voltage drop in NMOS 225N (insteadof PMOS 225R, in this case NMOS is used instead of PMOS 225 in thesensing circuitry). Like compensation circuit 500, compensation circuit1500 also includes current sources 1504-1506 for maintaining a constantcurrent through the NMOS 225N and adjustable resistor 1508, thusyielding a constant load at the NMOS 225N and a constant voltage dropacross the adjustable resistor 1508, respectively. In operation, theresistance of the resistor 1508 can be adjusted to achieve a desiredvoltage at the noninverting terminal of operational amplifier 1502, thusresulting in a constant voltage drop of a desired magnitude across NMOS225N. Accordingly, the adjustable resistor 1508 can be employed toachieve a desired load at NMOS 225N, despite variations in VDD and/orother parameters. The adjustable resistor 1508 for instance isimplemented as a binary or incremental-by-one-unit value by CMOS switchmuxing.

FIG. 3N illustrates further embodiments of a supply voltage compensationcircuit employing adjustable-level resistors consistent with one or moreaspects of the present innovations. More specifically, FIG. 3Nillustrates that other types of transistors can be employed as loadtransistors besides PMOS 225R. The compensation circuit 1600 of FIG. 3Nis similar to circuit 1500 of FIG. 3M, except that instead of PMOS 225R,transistor 1604 is used. Transistor 1604 can be any suitable transistor,but in particular it should be noted that the transistor 1604 can be anenhancement PMOS transistor, a native PMOS transistor, or a slightlydepleted PMOS transistor. It is known that these different PMOStransistors each have different threshold voltages. Typically,enhancement PMOS transistors have threshold voltages on the order of−0.6 V, while native PMOS transistors have threshold voltages on theorder of 0.0 V and slightly depleted PMOS transistors have thresholdvoltages on the order of −0.3 V. These different threshold voltagesgives a wider operating range for the tuning node VBA.

In addition to compensating for fluctuations in supply voltage, thepresent innovations encompass circuits configured to compensate forvoltage variations that take the PMOS 225 (i.e., the load element ofdata columns 202) out of its saturation region. That is, the innovationsmay include circuits for maintaining load elements in their saturationregion, i.e., operating in saturation mode. FIGS. 4A-4B are schematicdiagrams and detailed diagrams, respectively, of further supply voltagecompensation circuits in which load transistors operate in theirsaturation regions consistent with one or more aspects of the presentinnovations. Here, a compensation circuit 1700 maintains a gate-sourcevoltage difference that keeps the PMOS 225 in its saturation region, sothat current through the PMOS 225 does not vary, and a constant load ismore readily maintained.

The compensation circuit 1700 can be as simple as a voltage source 1702connected so as to maintain a specified voltage between the gate anddrain of PMOS 225, as shown. While the invention encompasses the use ofany suitable voltage source, one such configuration of voltage source1702 is shown in FIG. 4B, where a current source 1704 and PMOStransistor 1706 are connected in series between a voltage source node1708 and a ground terminal. The current source 1704 is connected betweenthe voltage source node 1708 at one end, and the gate of PMOS 225 andthe source terminal of PMOS 1706, at the other, as shown. The drain ofPMOS 1706 is grounded, while the gate of PMOS 1706 is connected to thedrain of PMOS 225. Configured in this manner, the current source 1704forces current through PMOS 1706, maintaining the voltage at the gate ofPMOS 225 at or above least the source-gate voltage difference of PMOS1706. PMOS 1706 can therefore be selected so that its source-gatevoltage difference keeps PMOS 225 in its saturation region.

FIGS. 5A-5B illustrate another embodiment of compensation circuits formaintaining data column load elements in their saturation regions. Here,an NMOS transistor 1804 is employed as the load element rather than PMOS225, and voltage source 1802 is connected between the drain and gate ofNMOS 1804, instead of the gate and source. An NMOS transistor may beused here because it provides faster operation, though is subject to abody effect. As with the voltage source 1702 above, the inventionencompasses the use of any suitable voltage source 1802 for maintaininga gate-drain voltage difference in NMOS 1804. FIG. 5B illustrates twoexamples of such a voltage source 1802. In the first example, thevoltage source 1802 includes a resistor 1806 and current source 1808connected in series between a voltage terminal 1810 (i.e., VDD) and aground terminal. The current from current source 1808 forces a specifiedvoltage drop across resistor 1806, placing a specified voltage at theoutput terminal (shown by the arrow) between resistor 1806 and currentsource 1808. In the second example, the voltage source 1802 includes afirst resistor 1812 and second resistor 1814 connected in series betweena voltage terminal 1816 (i.e., VDD) and a ground terminal. Theresistances of resistors 1812, 1814 are selected so as to yield aspecified voltage drop across resistor 1812 and thus a specified outputvoltage at the output terminal (shown by the arrow). In both examples,the output terminal is connected to the gate terminal of NMOS 1804, andapplies its output voltage to that gate terminal.

Each configuration of voltage source 1802 maintains a specifiedgate-drain voltage difference in NMOS 1804, the magnitude of which canbe selected to maintain the NMOS 1804 in its saturation region and thusa more constant load.

To further alleviate the problem of fluctuations in parameters such asVDD magnitude, the innovations here may also include comparator circuitsthat work at low VDD voltage values. FIGS. 6A-6B illustrate comparatorcircuits consistent with aspects of the present innovations that aredesigned to function at low VDD values, by employing transistorsutilizing a body effect. In FIG. 6A, a comparator circuit 1900 includesPMOS transistors 1902-1906, each connected serially to one oftransistors 1908-1912, respectively. The transistors 1908-1910 areconfigured as a current mirror, mirroring the same current through eachPMOS 1902-1904 to a ground terminal. The transistors 1902-1904 and1908-1910 constitutes a first gain stage. The first stage gain is equalto the ro*gmb, with gmb is the bulk transconductance of the transistor1902 and 1904 and ro is equivalent parallel output resistance of thetransistor 1910 and 1904. The transistor 1912 is configured as a commonsource amplifier with the transistor 1906 as a load. The transistor 1912and 1906 constitutes a second gain stage. The second stage gain is equalto the ro2*gm, with gm is the gate transconductance of the transistor1906 and 1912 and ro2 is equivalent parallel output resistance of thetransistor 1906 and 1912. The PMOS transistors 1902-1906 are eachconnected at one end to the VDD line, and at the other to acorresponding transistor 1908-1912. The output voltage line from thereference column 201 is input to the bulk terminal of PMOS 1902, whilethe output voltage line from the data column 202 is input to the bulkterminal of PMOS 1904. The voltage at the drain of PMOS 1906 is providedas the output of the comparator circuit 1900. In operation, voltagesignals from the reference column 201 and data column 202 are applied tothe bulk terminals of PMOS transistors 1902-1904, changing their bulktransconductances, and therefore their gains, according to the knownbody effect. As the same current is mirrored through each transistor1908-1912, differing voltages applied at the bulk terminals of PMOStransistors 1902-1904 thus result in a differing voltage appearing atthe output of the first stage and amplified further by the second stage.Accordingly, comparator 1900 produces a voltage signal that variesaccording to the difference in gains of the first and second stages. Ineffect the sensing circuitry provides a voltage output basing on thebulk transconductance differences basing on the reference memory outputlevel and the data memory output level.

The invention includes other comparator configurations, as well. In FIG.6B, a comparator circuit 2000 consistent with aspects of the presentinnovations includes four PMOS transistors 2002-2008 and additionaltransistors 2010-2016. The transistors 2010-2014 are configured ascurrent mirrors, and PMOS transistors 2002-2008 are connected eachbetween the VDD line at their source terminal, and a correspondingtransistor 2010-2016 at their drain terminal. The opposite terminals ofthe transistors 2010-2016 are grounded, as shown. The transistors2002-2004 and 2010-2012 are similarly to transistor 1902-1904 and1908-1910 of FIG. 6A. The transistor 2016 is similar to transistor 1912of FIG. 6A. The PMOS 2006 and 2008 are configured as a current mirrorand loading to transistor 2014 and 1016 respectively. The output voltageline from the reference column 201 is input to the bulk terminal of PMOS2002, while the output voltage line from the data column 202 is input tothe bulk terminal of PMOS 2004. The voltage at the source of PMOS 2008is provided as the output of the comparator circuit 2000. In operationthen, the gains of PMOS transistors 2002-2004 will differ with differingoutput voltages applied to their bulk terminals, resulting in a voltageat output node 2018 that varies according to the difference in theseoutput voltages.

It should be noted that the invention includes the use of the abovedescribed comparator circuits 1900, 2000 alone, and in conjunction withany one or more of the above described circuits, such as the prechargecircuit 600, the clamp circuit 700, and/or the buffer circuit 800.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the present inventionare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Many modifications and variations are possible in view of theabove teachings. For instance, the above described compensation circuitscan compensate for variations in many different parameters, includingvariations in VDD magnitude, temperature, and fabrication process, toproduce a fixed magnitude load. Similarly, the above describedcompensation circuits can be utilized alone, or in conjunction with anyone or more of the above described precharge circuits, buffer circuits,clamp circuits, or comparator circuits. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated.

1. A multilevel memory system configured to detect a voltage levelstored in a multilevel memory cell, comprising: a memory sensing circuitconnected to the multilevel memory cell and having a voltage supplyterminal, a load element connected between the voltage supply terminaland the multilevel memory cell, the load element providing a load forthe memory sensing circuit as a function of a voltage drop across atleast a portion of the load element and a current through the loadelement; a load compensation circuit connected to the load element andconfigured to maintain the load by compensating for a load variation ofthe load; and a precharge circuit in electrical communication with thememory sensing circuit, the precharge circuit configured to transmit aprecharge to the memory sensing circuit so as to reduce a settle time ofthe memory sensing circuit.
 2. The system of claim 1 wherein the loadelement is an enhancement PMOS transistor.
 3. The system of claim 1wherein the load element is a native PMOS transistor.
 4. The system ofclaim 1 wherein the load element is a slightly depleted PMOS transistor.5. The system of claim 1 wherein the load variation is a load variationdue to at least one of a variation in a magnitude of a voltage appliedat the voltage supply terminal, a variation in a temperature of the loadelement, or a variation in a fabrication of the load element.
 6. Thesystem of claim 1 wherein the load compensation circuit is furtherconfigured to maintain the load at a predetermined value.
 7. The systemof claim 1 wherein the load element is a PMOS transistor.
 8. The systemof claim 7 wherein the load compensation circuit is configured as avoltage follower circuit configured to maintain the load by maintaininga voltage drop across a portion of the PMOS transistor.
 9. A multi-levelmemory system configured to detect a voltage level stored in amultilevel memory cell, comprising: a memory sensing circuit connectedto the multilevel memory cell and configured to generate an outputvoltage signal corresponding to data stored in the multilevel memorycell; and a comparator circuit having a first transistor and a secondtransistor, the comparator circuit configured to compare the outputvoltage signal to a reference voltage signal according to a differencebetween a bulk gain of the first transistor and a bulk gain of thesecond transistor, the gain of the first transistor determined accordingto the reference voltage signal, and the gain of the second transistordetermined according to the output voltage signal; a precharge circuitin electrical communication with the memory sensing circuit, theprecharge circuit configured to transmit a precharge to the memorysensing circuit so as to reduce a settle time of the memory sensingcircuit.
 10. The system of claim 9 wherein the first transistor has afirst bulk terminal and the second transistor has a second bulkterminal, the comparator circuit receiving the reference voltage signalat the first bulk terminal so as to determine the gain of the firsttransistor according to a body effect at the first transistor, andreceiving the output voltage signal at the second bulk terminal so as todetermine the gain of the second transistor according to a body effectat the second transistor.
 11. The system of claim 10 wherein the firsttransistor is a first PMOS transistor having a first drain terminal anda first source terminal, and the second transistor is a second PMOStransistor having a second drain terminal and a second source terminal,the first and second source terminals in electrical communication with avoltage source, and the multilevel memory sensing system furthercomprising: a third PMOS transistor having a third source terminal inelectrical communication with the voltage source, and a third drainterminal; a current mirror circuit connected to the first and seconddrain terminals, and configured to mirror current through the first PMOStransistor into the second PMOS transistors; and an output terminal ofthe comparator circuit, the output terminal connected to the third drainterminal.
 12. The system of claim 10 wherein the first transistor is afirst PMOS transistor having a first drain terminal and a first sourceterminal, and the second transistor is a second PMOS transistor having asecond drain terminal and a second source terminal, the first and secondsource terminals in electrical communication with a voltage source, andthe multilevel memory sensing system further comprising: a third PMOStransistor having a third source terminal in electrical communicationwith the voltage source, and a third drain terminal; a fourth PMOStransistor having a fourth source terminal in electrical communicationwith the voltage source, and a fourth drain terminal; a current mirrorcircuit connected to the first, second, and third drain terminals, andconfigured to mirror current through the first PMOS transistor into thesecond, and third PMOS transistors; and an output terminal of thecomparator circuit, the output terminal connected to the fourth drainterminal.
 13. The system of claim 9 wherein the precharge circuitfurther comprises a plurality of resistors configured in electricalseries so as to transmit the precharge according to a ratio of theresistances of the resistors.
 14. The system of claim 9 wherein thememory system is non-volatile.
 15. The system of claim 9 wherein thememory sensing circuit is configured to generate an output voltagesignal corresponding to data stored in the multilevel memory cell, theoutput voltage having an associated noise, the system furthercomprising: a buffer circuit having an input terminal connected to thememory sensing circuit and configured to receive the output voltagesignal, and having an output terminal, the buffer circuit configured todecouple the output voltage signal from the associated noise so as toform a decoupled output voltage signal, and configured to transmit thedecoupled output voltage signal at the output terminal.
 16. Amulti-level memory system configured to detect a voltage level stored ina multilevel memory cell, comprising: a memory sensing circuit connectedto the multilevel memory cell and configured to generate an outputvoltage signal corresponding to data stored in the multilevel memorycell; and a comparator circuit having a first transistor and a secondtransistor, the comparator circuit configured to compare the outputvoltage signal to a reference voltage signal according to a differencebetween a bulk gain of the first transistor and a bulk gain of thesecond transistor, the gain of the first transistor determined accordingto the reference voltage signal, and the gain of the second transistordetermined according to the output voltage signal; a clamp circuit inelectrical communication with the memory sensing circuit, the clampcircuit configured to apply a clamp voltage to the memory sensingcircuit so as to maintain a voltage within the memory sensing circuit.17. The system of claim 16 wherein the clamp circuit further includes avoltage source terminal and a PMOS transistor coupled between thevoltage source terminal and the multilevel memory cell.
 18. The systemof claim 16 wherein the memory sensing circuit is configured to generatean output voltage signal corresponding to data stored in the multilevelmemory cell, the output voltage having an associated noise, the systemfurther comprising: a buffer circuit having an input terminal connectedto the memory sensing circuit and configured to receive the outputvoltage signal, and having an output terminal, the buffer circuitconfigured to decouple the output voltage signal from the associatednoise so as to form a decoupled output voltage signal, and configured totransmit the decoupled output voltage signal at the output terminal. 19.The system of claim 16 wherein the clamp circuit further includes avoltage source terminal and an NMOS transistor coupled between thevoltage source terminal and the multilevel memory cell.
 20. The systemof claim 16 wherein the memory system is non-volatile.
 21. The system ofclaim 16 wherein the first transistor has a first bulk terminal and thesecond transistor has a second bulk terminal, the comparator circuitreceiving the reference voltage signal at the first bulk terminal so asto determine the gain of the first transistor according to a body effectat the first transistor, and receiving the output voltage signal at thesecond bulk terminal so as to determine the gain of the secondtransistor according to a body effect at the second transistor.
 22. Thesystem of claim 21 wherein the first transistor is a first PMOStransistor having a first drain terminal and a first source terminal,and the second transistor is a second PMOS transistor having a seconddrain terminal and a second source terminal, the first and second sourceterminals in electrical communication with a voltage source, and themultilevel memory sensing system further comprising: a third PMOStransistor having a third source terminal in electrical communicationwith the voltage source, and a third drain terminal; a current mirrorcircuit connected to the first and second drain terminals, andconfigured to mirror current through the first PMOS transistor into thesecond PMOS transistors; and an output terminal of the comparatorcircuit, the output terminal connected to the third drain terminal. 23.The system of claim 21 wherein the first transistor is a first PMOStransistor having a first drain terminal and a first source terminal,and the second transistor is a second PMOS transistor having a seconddrain terminal and a second source terminal, the first and second sourceterminals in electrical communication with a voltage source, and themultilevel memory sensing system further comprising: a third PMOStransistor having a third source terminal in electrical communicationwith the voltage source, and a third drain terminal; a fourth PMOStransistor having a fourth source terminal in electrical communicationwith the voltage source, and a fourth drain terminal; a current mirrorcircuit connected to the first, second, and third drain terminals, andconfigured to mirror current through the first PMOS transistor into thesecond, and third PMOS transistors; and an output terminal of thecomparator circuit, the output terminal connected to the fourth drainterminal.
 24. A multilevel memory system configured to detect a voltagelevel stored in a multilevel memory cell, comprising: a memory sensingcircuit connected to the multilevel memory cell and having a voltagesupply terminal, a load element connected between the voltage supplyterminal and the multilevel memory cell, the load element providing aload for the memory sensing circuit as a function of a voltage dropacross at least a portion of the load element and a current through theload element; a load compensation circuit connected to the load elementand configured to maintain the load by compensating for a load variationof the load; and a clamp circuit in electrical communication with thememory sensing circuit, the clamp circuit configured to apply a clampvoltage to the memory sensing circuit so as to maintain a voltage withinthe memory sensing circuit.
 25. The system of claim 24 wherein the loadelement is an enhancement PMOS transistor.
 26. The system of claim 24wherein the load element is a native PMOS transistor.
 27. The system ofclaim 24 wherein the load element is a slightly depleted PMOStransistor.
 28. The system of claim 24 wherein the load variation is aload variation due to at least one of a variation in a magnitude of avoltage applied at the voltage supply terminal, a variation in atemperature of the load element, or a variation in a fabrication of theload element.
 29. The system of claim 24 wherein the load compensationcircuit is further configured to maintain the load at a predeterminedvalue.
 30. The system of claim 24 wherein the load element is a PMOStransistor.
 31. The system of claim 30 wherein the load compensationcircuit is configured as a voltage follower circuit configured tomaintain the load by maintaining a voltage drop across a portion of thePMOS transistor.